Devices in which a liquid substance passes through a membrane are commonly used for separating substances of a liquid composition. In connection with water treatment, such devices are used for desalination purposes by separation of dissolved salts from water, and for water purification in general e.g. by separation of contaminants such as bacteria and virus from water. Similar devices are used for removing liquid from fruit juice to provide a concentrate thereof and in general for removing substances from a liquid or for concentrating substances in a liquid.
In general, the separation structure receives a liquid composition comprising various substances, in the following referred to as inlet liquid, and delivers two different liquids each having different contents of one of more substances. Certainly, both of these two different liquids may be valuable. Often, however, one of the two liquids is a desired liquid, and the other one is to be disposed of. To distinguish the two liquids, the following description will be based on the term “the first liquid” for the liquid which has not passed through the membrane, and “the second liquid” for the liquid which has passed through the membrane.
In general, it is desired that as much liquid as possible permeates through the membrane. This may be achieved by increasing the pressure difference across the membrane, i.e. by increasing the pressure of the inlet liquid to a point which is close to a maximum limit prescribed for the membrane. In a desalination plant working in accordance with the principles of reverse osmosis, the inlet water (saltwater) is typically pressurised to approximately 60-70 bar depending upon the salt concentration. At this pressure, the saltwater enters a separation structure with a reverse osmosis membrane which is permeable to pure water. Since the utilization rate is typically 25-35%, 65-75% of the saltwater is rejected from the system as a residual product of the process (the first fluid). This rejected water has a higher salt concentration than the received saltwater and will in the following be referred to as brine. During returning of the brine, it undergoes a pressure drop of 60-70 bar and the energy thereby released corresponds to 65-75 percent of the energy which has been consumed by the pump which initially raised the pressure of the saltwater. In order to recover the energy which is released during the returning of the seawater, a pressure recovery unit is often inserted after the separation structure.
A reverse osmosis system is shown e.g. in U.S. Pat. No. 5,482,441.
The pressure recovery unit not only recovers the pressure and thus saves energy; it also provides a sufficient back pressure in the outlet of the first fluid.
A general problem with the existing devices is that fouling in the membrane increases the resistance against permeation of the liquid through the membrane. As a consequence, a fixed pressure of the inlet liquid provides a ratio between the first liquid and the second liquid which changes over time. This characteristic of the known devices is undesired. Firstly, the loading and thus the wear of the often very expensive membrane increases over time, and secondly, a specific consumption of the first and/or second liquid requires over-dimensioning of the device when the membrane is new.